Remove semi-functional .selector calculation from .pushStack()

Description

Per discussion with ben_alman, and related to #7389 which consensus says won't happen.

The .selector property can't be reliable since there are simple ways to construct a set that can't be represented by a CSS selector. We've documented those shortcomings in the API docs for .live() and .selector but it's still a frequent source of questions and errors.

It would simplify the code for .pushStack() and its callers if it didn't try to maintain the .selector property at all, which is still in keeping with our documented use of .live() immediately after a $() set creation.

In addition we should deprecate non-core use of .selector and remove the documentation from the API site eventually.

All to be done over several versions, with sufficient advance warning in the docs before breaking anything.